Radiobuttonlist validator

I am new in programming in visual basic and am working on a project in VB web in visual studio 2008. I have a group of radio buttons, one is a and the other is b. I when the user doesn't select one of the buttons i need a required validate error to tell the user to select a choice of a or b. I don't want to use a message label. The book we are using doesn't explain all the details.Thanks for your helpPat